1. Field of the Invention
This invention relates to a hydraulic type antivibration device, more particularly to the one capable of exhibiting a good vibration control effect to both low frequency, large amplitude vibrations and high frequency, fine amplitude vibrations.
2. Description of Related Art
Hitherto as a hydraulic antivibration device, for example, installed between an automotive transmission and a car body frame, there is known such an antivibration device that is made up of a first attachment part, a cylindrical second attachment part, a vibration isolating base made of rubber-like elastomer interconnecting the former two, a diaphragm attached to the second attachment fitting to form a liquid-filled chamber between the diaphragm and the vibration isolating base, a partition comparting the liquid-filled chamber into a first liquid chamber on the vibration isolating base side and a second liquid chamber on the diaphragm side, and an orifice putting the first liquid chamber and the second liquid chamber into communication with each other, and wherein the partition includes an elastic partition membrane and a pair of displacement-regulating members for regulating the displacement amount of the elastic partition membrane from both sides of its both membrane faces, the pair of the displacement-regulating members being pierced by respective openings. In this antivibration device, when a large amplitude vibration is generated owing to irregularity of the road face during travelling, the liquid flows through the orifice between both liquid chambers and the vibration is dampened by reason of the liquid fluidization effect. On the other hand, when a fine amplitude vibration is generated, the liquid does not flow between both liquid chambers, but the elastic partition membrane undergoes reciprocating deformation and absorbs the internal pressure of the first liquid chamber to dampen the vibration.
In this hydraulic type of antivibration device, when the elastic partition membrane collides with the displacement-regulating members, strange or unusual sounds are liable to be generated. In the circumstances, as disclosed in U.S. Pat. No. 5,443,245 (Patent Reference 1), in order to diminish the contact area of the elastic partition membrane with grid-like displacement-regulating members, a technology of providing the displacement-regulating members with radial ribs is proposed. According to this proposed construction, strange sounds can be reduced to some degree, but the contact area between the elastic partition membrane and the displacement-regulating members cannot be diminished so much, so that strange sounds cannot be suppressed sufficiently. Moreover the area of the opening for transmitting the liquid pressure of the first liquid chamber to the elastic partition membrane is narrowed by the space amount of the grids of the displacement-regulating members, and consequently, it is hard to absorb the internal pressure of the first liquid chamber.
Further, in JP Patent Publication 2003-294078 A (Patent Reference 2) it is disclosed that in order to prevent leakage of liquid between through-holes at grid-like displacement-regulating members, spider web-like ribs surrounding the respective through-holes over a full periphery thereof are provided on faces of the elastic partition membrane. In this construction, because the ribs surround the respective through-holes over the full periphery thereof, the contact area of the elastic partition membrane with the displacement-regulating members is large, so that the dynamic load increases largely with the result that strange sounds are likely to be generated.
Furthermore in JP Patent Publication 2003-74617 A (Patent Reference 3) there is disclosed the construction that in order to suppress the tendency toward a high dynamic spring characteristic in a higher frequency range than the tuning frequency of the orifice while ensuring sufficiently the vibration controlling effect by the orifice, the elastic partition membrane is formed of a central thick-walled section, a thin-walled section at the outer periphery of the former, and a stationary section at the outer peripheral margin thereof, and an outer peripheral margin of the thick-walled section and the stationary section are pinched by a pair of displacement-regulating members over a full periphery of both in a compressed state in the thickness direction. In this case, the outer peripheral margin of the thick-walled section is pinched by the displacement-regulating members over a full periphery thereof, and consequently, the effective area of the opening for transmitting the liquid pressure of the first liquid chamber to the elastic partition membrane is small and the thick-walled section is difficult to move. As a consequence, a dynamic spring constant in a high frequency range (e.g., on the order of 100 Hz) is increased and a buzzing noise is prone to be generated.
Meanwhile, from U.S. Pat. No. 5,104,100 (Patent Reference 4) such a construction is known that a plurality of small protrusions are provided on either face (front face and reverse face) of an annular thick-walled section of the elastic partition membrane equally in the circumferential direction in a distributed manner so that the small protrusions may abut against a pair of the displacement-regulating members. However, this Reference is distinct from this invention regarding the fundamental structure of the elastic partition membrane in that slits or cuts are provided in the central area thereof and besides, the thick-walled section is provided with small protrusions abutting against the displacement-regulating members. Thus this Reference is different from this invention in the position relation of the elastic partition membrane to the displacement-regulating members as well as the constitution of the elastic partition membrane.
Further JP UM Application Publication 64-36737 (Patent Reference 5) describes that the elastic partition membrane is constructed of a central thick-walled section and a thin-walled section around it, but in the antivibration device of this Reference, no displacement-regulating members are provided, and consequently, no mention is made of ribs at the elastic partition membrane, still less of a position relation between the elastic membrane and displacement-regulating members.